The present invention relates generally to magnetic resonance imaging (MRI) systems and, more particularly, to minimizing temperature variation throughout a cooling system of a gradient coil of an MRI system leading to lower coil temperatures. The present invention is also related to minimizing the size and components of a cooling system for a gradient coil of an MRI system, and reducing pumping requirements for coolant within a gradient coil of an MRI system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Many MR systems use gradient coils in which a coolant flows therethrough. A chiller utilizing water or a water and ethylene glycol mixture, or a coolant having dielectric or non-dielectric fluid mixtures, is typically used for removing heat from gradient coils and from both primary and shield parts. As power requirements for MR systems increase, the complexity of the thermal design has increased as well. Typically an MR system can generate significant amounts of heat, which can be 15-20 kW, and future devices are expected to increase up to 100 kW or more. Additionally, as a coolant flows through the cooling channels of MR gradient coils, the temperature rise of the coolant can be substantial, which in turn could cause hot spots in the MR gradient coils and potentially degrade system performance. Because the coolant is distributed throughout a series of sub-circuits, flow can stagnate in portions of the overall flow circuit, or flow non-uniformly due to pressure non-uniformities, thus depriving portions of the gradient coils and further causing hot spots to form within the gradient coils. Common solutions to increasing cooling requirements within MR systems are to either provide more cooling circuitry or to increase the flow rate within the gradient coils. Both solutions can have practical limitations for increasing cooling requirements, such as additional design complexity, pressure drop, and reliability concerns.
It would therefore be desirable to have a system and method capable of having a large cooling capacity and having a uniform pressure drop throughout an MR gradient cooling system. It would further be desirable to have a system and method capable of having a uniform temperature of a coolant throughout an MR gradient cooling system.